Oxygen sensing cells are well known in the prior art. Their purpose is to monitor the oxygen level in a gas or liquid. These cells are electrochemical in their operation and are arranged to generate an output signal whenever the oxygen level in the sample being monitored reaches a predetermined threshold value. The output signal triggers an electronic circuit for activating an alarm or adjusting a process. For example, the cells may be used for detecting potentially dangerous levels of oxygen in a combustible gas, or for on-line monitoring of oxygen in a chemical or petrochemical plant process stream.
In general, there are two types of oxygen sensing cells using electrochemical techniques. One is the galvanometric cell which generates its own signal current whenever oxygen molecules pass through its membrane. These cells are similar to miniature fuel cells. The other type of sensing cell is polarographic, that is, it is polarized by an external voltage source.
Construction wise, these polarographic oxygen sensing cells generally consist of a body having a main cavity containing a suitable electrolyte. The body may be closed at one end and covered at the other end by a stretched membrane. This membrane, which may be made of polyethylene or other suitable material, is liquid-impermeable but gas-permeable. It thus seals the liquid electrolyte within the cell, but permits the gas being sampled to pass into the cell. An anode is mounted within the cavity and cooperates with a cathode mounted on the cell body internally of the membrane. By way of example, the anode may be made of silver, the cathode of gold, and the electrolyte may consist of a potassium chloride solution. Suitable respective conductors are carried by the body for connecting the anode and cathode to external circuitry. A positive bias is placed on the anode, thereby polarizing the cell, and the output signal is developed between the cathode and ground to obtain the desired measurement.
While generally satisfactory for the purposes intended, these prior art designs and constructions have certain inherent disadvantages or deficiencies; and thus the need has continued to exist for an improved oxygen sensing cell, one which has a substantially improved long-term stability, dynamic range and life, as well as other desirable characteristics.